1 inotify
2 a powerful yet simple file change notification system
3 4 5 6Document started 15 Mar 2005 by Robert Love <rml@novell.com>
7 8 9(i) User Interface
10 11Inotify is controlled by a set of three system calls and normal file I/O on a
12returned file descriptor.
13 14First step in using inotify is to initialise an inotify instance:
15 16 int fd = inotify_init ();
17 18Each instance is associated with a unique, ordered queue.
19 20Change events are managed by "watches". A watch is an (object,mask) pair where
21the object is a file or directory and the mask is a bit mask of one or more
22inotify events that the application wishes to receive. See <linux/inotify.h>
23for valid events. A watch is referenced by a watch descriptor, or wd.
24 25Watches are added via a path to the file.
26 27Watches on a directory will return events on any files inside of the directory.
28 29Adding a watch is simple:
30 31 int wd = inotify_add_watch (fd, path, mask);
32 33Where "fd" is the return value from inotify_init(), path is the path to the
34object to watch, and mask is the watch mask (see <linux/inotify.h>).
35 36You can update an existing watch in the same manner, by passing in a new mask.
37 38An existing watch is removed via
39 40 int ret = inotify_rm_watch (fd, wd);
41 42Events are provided in the form of an inotify_event structure that is read(2)
43from a given inotify instance. The filename is of dynamic length and follows
44the struct. It is of size len. The filename is padded with null bytes to
45ensure proper alignment. This padding is reflected in len.
46 47You can slurp multiple events by passing a large buffer, for example
48 49 size_t len = read (fd, buf, BUF_LEN);
50 51Where "buf" is a pointer to an array of "inotify_event" structures at least
52BUF_LEN bytes in size. The above example will return as many events as are
53available and fit in BUF_LEN.
54 55Each inotify instance fd is also select()- and poll()-able.
56 57You can find the size of the current event queue via the standard FIONREAD
58ioctl on the fd returned by inotify_init().
59 60All watches are destroyed and cleaned up on close.
61 62 63(ii)
64 65Prototypes:
66 67 int inotify_init (void);
68 int inotify_add_watch (int fd, const char *path, __u32 mask);
69 int inotify_rm_watch (int fd, __u32 mask);
70 71 72(iii) Kernel Interface
73 74Inotify's kernel API consists a set of functions for managing watches and an
75event callback.
76 77To use the kernel API, you must first initialize an inotify instance with a set
78of inotify_operations. You are given an opaque inotify_handle, which you use
79for any further calls to inotify.
80 81 struct inotify_handle *ih = inotify_init(my_event_handler);
82 83You must provide a function for processing events and a function for destroying
84the inotify watch.
85 86 void handle_event(struct inotify_watch *watch, u32 wd, u32 mask,
87 u32 cookie, const char *name, struct inode *inode)
88 89 watch - the pointer to the inotify_watch that triggered this call
90 wd - the watch descriptor
91 mask - describes the event that occurred
92 cookie - an identifier for synchronizing events
93 name - the dentry name for affected files in a directory-based event
94 inode - the affected inode in a directory-based event
95 96 void destroy_watch(struct inotify_watch *watch)
97 98You may add watches by providing a pre-allocated and initialized inotify_watch
99structure and specifying the inode to watch along with an inotify event mask.
100You must pin the inode during the call. You will likely wish to embed the
101inotify_watch structure in a structure of your own which contains other
102information about the watch. Once you add an inotify watch, it is immediately
103subject to removal depending on filesystem events. You must grab a reference if
104you depend on the watch hanging around after the call.
105 106 inotify_init_watch(&my_watch->iwatch);
107 inotify_get_watch(&my_watch->iwatch); // optional
108 s32 wd = inotify_add_watch(ih, &my_watch->iwatch, inode, mask);
109 inotify_put_watch(&my_watch->iwatch); // optional
110 111You may use the watch descriptor (wd) or the address of the inotify_watch for
112other inotify operations. You must not directly read or manipulate data in the
113inotify_watch. Additionally, you must not call inotify_add_watch() more than
114once for a given inotify_watch structure, unless you have first called either
115inotify_rm_watch() or inotify_rm_wd().
116 117To determine if you have already registered a watch for a given inode, you may
118call inotify_find_watch(), which gives you both the wd and the watch pointer for
119the inotify_watch, or an error if the watch does not exist.
120 121 wd = inotify_find_watch(ih, inode, &watchp);
122 123You may use container_of() on the watch pointer to access your own data
124associated with a given watch. When an existing watch is found,
125inotify_find_watch() bumps the refcount before releasing its locks. You must
126put that reference with:
127 128 put_inotify_watch(watchp);
129 130Call inotify_find_update_watch() to update the event mask for an existing watch.
131inotify_find_update_watch() returns the wd of the updated watch, or an error if
132the watch does not exist.
133 134 wd = inotify_find_update_watch(ih, inode, mask);
135 136An existing watch may be removed by calling either inotify_rm_watch() or
137inotify_rm_wd().
138 139 int ret = inotify_rm_watch(ih, &my_watch->iwatch);
140 int ret = inotify_rm_wd(ih, wd);
141 142A watch may be removed while executing your event handler with the following:
143 144 inotify_remove_watch_locked(ih, iwatch);
145 146Call inotify_destroy() to remove all watches from your inotify instance and
147release it. If there are no outstanding references, inotify_destroy() will call
148your destroy_watch op for each watch.
149 150 inotify_destroy(ih);
151 152When inotify removes a watch, it sends an IN_IGNORED event to your callback.
153You may use this event as an indication to free the watch memory. Note that
154inotify may remove a watch due to filesystem events, as well as by your request.
155If you use IN_ONESHOT, inotify will remove the watch after the first event, at
156which point you may call the final inotify_put_watch.
157 158(iv) Kernel Interface Prototypes
159 160 struct inotify_handle *inotify_init(struct inotify_operations *ops);
161 162 inotify_init_watch(struct inotify_watch *watch);
163 164 s32 inotify_add_watch(struct inotify_handle *ih,
165 struct inotify_watch *watch,
166 struct inode *inode, u32 mask);
167 168 s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
169 struct inotify_watch **watchp);
170 171 s32 inotify_find_update_watch(struct inotify_handle *ih,
172 struct inode *inode, u32 mask);
173 174 int inotify_rm_wd(struct inotify_handle *ih, u32 wd);
175 176 int inotify_rm_watch(struct inotify_handle *ih,
177 struct inotify_watch *watch);
178 179 void inotify_remove_watch_locked(struct inotify_handle *ih,
180 struct inotify_watch *watch);
181 182 void inotify_destroy(struct inotify_handle *ih);
183 184 void get_inotify_watch(struct inotify_watch *watch);
185 void put_inotify_watch(struct inotify_watch *watch);
186 187 188(v) Internal Kernel Implementation
189 190Each inotify instance is represented by an inotify_handle structure.
191Inotify's userspace consumers also have an inotify_device which is
192associated with the inotify_handle, and on which events are queued.
193 194Each watch is associated with an inotify_watch structure. Watches are chained
195off of each associated inotify_handle and each associated inode.
196 197See fs/notify/inotify/inotify_fsnotify.c and fs/notify/inotify/inotify_user.c
198for the locking and lifetime rules.
199 200 201(vi) Rationale
202 203Q: What is the design decision behind not tying the watch to the open fd of
204 the watched object?
205 206A: Watches are associated with an open inotify device, not an open file.
207 This solves the primary problem with dnotify: keeping the file open pins
208 the file and thus, worse, pins the mount. Dnotify is therefore infeasible
209 for use on a desktop system with removable media as the media cannot be
210 unmounted. Watching a file should not require that it be open.
211 212Q: What is the design decision behind using an-fd-per-instance as opposed to
213 an fd-per-watch?
214 215A: An fd-per-watch quickly consumes more file descriptors than are allowed,
216 more fd's than are feasible to manage, and more fd's than are optimally
217 select()-able. Yes, root can bump the per-process fd limit and yes, users
218 can use epoll, but requiring both is a silly and extraneous requirement.
219 A watch consumes less memory than an open file, separating the number
220 spaces is thus sensible. The current design is what user-space developers
221 want: Users initialize inotify, once, and add n watches, requiring but one
222 fd and no twiddling with fd limits. Initializing an inotify instance two
223 thousand times is silly. If we can implement user-space's preferences
224 cleanly--and we can, the idr layer makes stuff like this trivial--then we
225 should.
226 227 There are other good arguments. With a single fd, there is a single
228 item to block on, which is mapped to a single queue of events. The single
229 fd returns all watch events and also any potential out-of-band data. If
230 every fd was a separate watch,
231 232 - There would be no way to get event ordering. Events on file foo and
233 file bar would pop poll() on both fd's, but there would be no way to tell
234 which happened first. A single queue trivially gives you ordering. Such
235 ordering is crucial to existing applications such as Beagle. Imagine
236 "mv a b ; mv b a" events without ordering.
237 238 - We'd have to maintain n fd's and n internal queues with state,
239 versus just one. It is a lot messier in the kernel. A single, linear
240 queue is the data structure that makes sense.
241 242 - User-space developers prefer the current API. The Beagle guys, for
243 example, love it. Trust me, I asked. It is not a surprise: Who'd want
244 to manage and block on 1000 fd's via select?
245 246 - No way to get out of band data.
247 248 - 1024 is still too low. ;-)
249 250 When you talk about designing a file change notification system that
251 scales to 1000s of directories, juggling 1000s of fd's just does not seem
252 the right interface. It is too heavy.
253 254 Additionally, it _is_ possible to more than one instance and
255 juggle more than one queue and thus more than one associated fd. There
256 need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
257 process can easily want more than one queue.
258 259Q: Why the system call approach?
260 261A: The poor user-space interface is the second biggest problem with dnotify.
262 Signals are a terrible, terrible interface for file notification. Or for
263 anything, for that matter. The ideal solution, from all perspectives, is a
264 file descriptor-based one that allows basic file I/O and poll/select.
265 Obtaining the fd and managing the watches could have been done either via a
266 device file or a family of new system calls. We decided to implement a
267 family of system calls because that is the preferred approach for new kernel
268 interfaces. The only real difference was whether we wanted to use open(2)
269 and ioctl(2) or a couple of new system calls. System calls beat ioctls.
270 271